Modern communication networks often show a “decomposed” or layered architecture, in which the call and session control layer and the media plane layer are handled by different instances that are typically realized as separated physical nodes. Typically the nodes of the call and session control layer are referred to as media gateway controllers and the nodes of the media plane layer are referred to as media gateways.
A protocol widely used in the control layer for controlling sessions consisting of one or several media streams is the Session Initiation Protocol SIP being specified in the Internet Engineering Task Force document “IETF RFC 3262, Session Initiation Protocol” in the following being referred to as RFC 3262.
A further protocol defined by the IETF, the Session Description Protocol SDP specified in the IETF document RFC 4566, is used in a variety of networks and communications systems for describing multimedia sessions, e.g. for the purpose of session announcement, session invitation and other forms of multimedia session initiation. Examples of such networks are the IP Multimedia Subsystem—IMS- and the Multimedia Telephony Service—MMTel-. The SDP is thereby used in combination with the SIP in a way that SDP messages or packages are embedded into SIP.
Since its origins, SDP has evolved with new capabilities to respond to the needs of new applications, as corresponds to a constantly growing use of the protocol. A capability to group different media has been specified for different applications in various IETF documents. By means of media grouping in SDP, a particular relationship between two or more media streams can be indicated.
The framework for grouping of media is specified in the IETF document RFC 5888 that also specifies specific applications for the use of this capability, and further in the IETF document RFC 4588 titled “The Session Description Protocol (SDP) Grouping Framework”.
According to the RFC 5888, a plurality of media streams may be grouped together. Depending on the type (semantics) of the grouping, certain actions which would otherwise be handled individually per stream may be required on the complete group of streams. In the following, some examples are given:
An announcement (video and/or audio) shall be sent simultaneously to two grouped (video and/or audio) media streams
When several grouped media streams traverse a node, for example a media mixer, the input and output stream end points of the node may have to be connected in such a way that several grouped media streams at one end point are mapped into one single media stream at the other end point.
Statistics: for some streams that are grouped together according to the “BUNDLE” semantics it may be required to collect statistics (e.g. received octets) for all of them together, rather than for each of them individually.
Events: for some streams that are grouped together (e.g. using so-called flow Identification, FID, semantics), it may be desired to arm the detection of incoming digits (eDTMF or telephony events) simultaneously in all grouped streams.
One example for a protocol to be used between the control nodes and the media plane nodes is specified in ITU-T Recommendation H.248.1, titled Gateway Control Protocol (current Version 3), in the following also being referred to as media gateway control protocol or simply as H.248.
In a decomposed gateway, the actions above are controlled by MGC instructions e.g. instructions according to the H.248 protocol.
Patent application PCT/EP2011/061462 of the same applicant discloses a method to initiate a media session within a communications network involving a plurality of media streams, wherein the communications network comprises a plurality of media control nodes (media gateway controllers) and a plurality media resource nodes (media gateways) being controlled by each one of the media control nodes. Thereto, a certain media control nodes instructs a media resource node being controlled by that media control nodes to associate or group two or more of the media streams, e.g. a subset of all streams handled by a termination of the media resource node.
This allows a control layer node (a media gateway controller or MRFC) to instruct a node of the media layer (a media gateway or a MRFP), by means of a corresponding session description, to create appropriate media stream sinks and sources (such logical entities also being referred to as terminations within the H.248 terminology) for handling multiple media streams and grouping corresponding two or more of these media streams. Thus controlling a grouping (or association) of different media streams of multimedia applications like videoconference and so-called telepresence might be accomplished involving a plurality of media gateway controllers and a corresponding plurality of media gateways.
Example for use cases are:                Synchronized play out of a video and an audio stream (lip-synchonization, LS) or of an audio stream and its simultaneous translation (LS semantics are defined in RFC 5888).        Use of a redundancy media stream for error protection purposes (the original stream and the redundancy stream may use the FID media grouping semantics, as defined in RFC 4888).        Use of one or more Forward Error Correction streams (the original stream and the forward error correction stream(s) may use the FEC media grouping semantics, as defined in RFC 4756).        The FID semantics may also be used to group multiple audio streams which use different audio codecs. Only one of them is used at a time. The streams with the ones not used remain silent during that time. In another use of the FID semantics, if two of these streams use the same audio codec, a replica of the media is sent is sent in the stream with the same codec (as specified in RFC 5888).        Two or more media streams may use the same source and destination IP addresses and ports. These media grouping semantics is called BUNDLE and is e.g. being specified in the draft-ietf-mmusic-sdp-bundle-negotiation IETF draft.        Multiple video streams with the same content, but with different resolutions may be grouped together with a simulcast media grouping.        
However, not always all streams associated to one termination within the gateway are to be grouped. There may be situations wherein only a subset of such media streams shall be grouped. However, current protocols for the control of media gateways do not allow commanding certain actions, such as those previously listed regarding playout of signals, detection of events, collection of statistics, on a stream group when such partial grouping exists. Furthermore, for some grouping semantics the realization of a signal, event or statistics in the group of streams may not be identical to the realization of such signal, event or statistic in every individual stream of the group. In such case, current protocols also do not allow commanding such signal, event or statistics in the group of streams. Furthermore, current protocols do not allow commanding the establishment of certain forms of connectivity if the same grouping is not defined at the other side of the connection, i.e. in the other termination.
Specifically, H.248 does not allow applying a signal simultaneously to several media streams in a termination unless it is applied to all media streams of the termination. Further, H.248 does not allow arming an event simultaneously in several streams in a termination unless it is armed in all media streams of the termination. There is further no mechanism to combine (aggregate) statistics on several streams in a termination (unless it is done on all streams of the termination).
There is no mechanism to connect both-way or one-way two or more streams at one side with one single stream at the other side (unless the node applies multiplexing). The mechanism used in H.248 to specify the connectivity between the stream end points is the topology. It may be noticed that H.248 supports the mixer case where several stream end points are connected to each other, normally with both-way topology, but in that case every end point handles the same number of streams.